The present invention relates to an image forming apparatus which forms an image on the rotating photoreceptor through a digital system.
In an electrophotographic copying machine or printer, an electrostatic latent image is formed on the surface of a rotating cylindrical photoreceptor or of a running belt-shaped photoreceptor, then, the formed electrostatic latent image is developed with toner, and the toner image thus developed is transferred onto and fixed on a recording sheet to obtain an image.
Now, let it be assumed that a driving roller which makes the cylindrical photoreceptor of the apparatus representing a photoreceptor drum to rotate and makes the belt-shaped photoreceptor to run is called a rotational body.
When there is caused speed variation (speed fluctuation) on a photoreceptor which is supposed to rotate at a constant speed, small pitch banding phenomenon takes place, and the jitters and image irregularities are caused on the outputted image. This appears especially conspicuously in an electrophotographic technology of a digital system wherein writing on a photoreceptor is conducted through scanning by a semiconductor laser, and speed variation in the sub-scanning direction in the writing system is caused by the speed variation of rotation of the photoreceptor, causing delicate deviation of writing lines in their intervals, resulting in a factor to lower image quality sharply.
With regard to technologies for improving accuracy in driving a rotational body which is supposed to rotate at a constant speed, there are many proposals which are roughly divided into the following two categories.
One of the two categories is one wherein a flywheel is incorporated in the driving system, and TOKKAIHEI Nos. 7-281500 and 8-202205 disclose those wherein a conventional flywheel is easily mounted or dismounted. Further, TOKKAIHEI Nos. 6-130872, 6-130874, 7-302025 and 8-202206 disclose technologies wherein a flywheel is provided in a rotational body. In addition, TOKKAIHEI Nos. 8-63041, 8-115041, and 8-220966 disclose technologies wherein frequency response of a rotational body are detected and thereby, moment of inertia of a flywheel is normalized in connection with vibrating frequency.
The other of the two categories is one wherein vibration in the rotational direction of the drive transmitting system is absorbed by using gears and timing belt pulleys in which elastic members are provided on the half way of the rotational body driving system. Concrete examples are shown in TOKKAIHEI Nos. 6-249321, 6-294453, 7-325445, 7-325446 and 8-54047.
As stated above, in prior art, use of a flywheel has been the most effective technical means for improving accuracy in driving a rotational body, but there has been a theoretical problem that a large apparatus is required and large torque is also required in the rise of rotation. Further, since the flywheel itself is one to reduce rotational vibration by its rotational kinetic energy, when obtaining its effect in the case of low speed rotation of a rotational body, it is necessary to use a flywheel having a larger diameter compared with the rotational body. Therefore, for the purpose of avoiding a large-sized apparatus, one has had to be contented with a functional limit even when providing a flywheel in the rotational body.
It has become popular recently to obtain a natural frequency of a driving system for a rotational body and thereby to design the driving system taking the relation with vibrating frequency into consideration, and to obtain frequency response of a rotational system focusing on a natural frequency and thereby to change a form of frequency response, namely a form of transfer functions by the design of inertial quantity to change the peak position so that the driving system may be optimum. The most serious problem in this case is that a diameter of the flywheel is required to be large or the flywheel is required to be heavy in weight, naturally in the case of a single rotational body driving system, when considering to move a natural frequency to a low frequency area. This means that, when natural frequency f of basic frequency is represented by ##EQU1## the value of natural frequency f is made small by making moment of inertia I of the rotational body to be large. (K in this case represents torsional rigidity of the driving system.)
On the other hand, improving the driving accuracy by providing elastic members on the half way of the driving system means that a vibration component in the rotational direction generated in the driving system is converted into heat in the elastic members to be diffused. Since there are no concepts of frequency response and transfer function in this case, effects of the elastic members can not be predicted depending upon vibrating frequency of the generated vibration and the structure of the driving system, resulting in different levels of effects.
However, in development of an image forming apparatus of a digital system, reproducibility of one dot line by laser writing is required strictly as performances are improved and accuracy required for the driving system has become strict rapidly. The accuracy required in this case is on the level wherein uniformity of laser writing in the sub-scanning direction can be guaranteed in connection with visual sensitivity of a visual system, and with the trend of high density recording such as 600 dpi-2400 dpi, there is required highly accurate driving of a rotational body having no speed fluctuation and satisfying a high level in which a human being can not recognize small pich banding.
For the reasons mentioned above, there are generally employed highly accurate gears, exclusive driving, and a large-sized flywheel in the scope of prior art. However, when employing a flywheel, there is no way to avoid heavy weight and a large apparatus. For providing a printer of an electrophotographic system, too, there has been proposed a structure to transmit gear driving force to a rotational body such as a photoreceptor drum through elastic members because of necessity to materialize the highly accurate driving structure which is light in weight, compact in size and low in cost. However, this technology which employs only elastic members has had a problem that a gain in a resonance area is large, and speed fluctuation in the vicinity of the resonance area and load variation adversely affect to a great extent to worsen small pich banding although the technology has shown great effects to reduce speed fluctuation which is higher than a natural frequency.